Long years ago I used a program called "switch" from NCH Software to shrink voice recording files drastically without significant loss of clarity. My recollection is the original MP3 recordings (from a Sony pocket voice recorder) ran about 10 MB per minute, and the converted files about 10 minutes per MB, in other words a 100-fold reduction in file size without noticeable loss of quality (and yes, I know you couldn't do this with nature or music recordings, but the human voice in conversation has limited bandwidth).
That was a good many years ago and now, needing to do the same thing again, I have forgotten how I did it. Does anyone have words of wisdom here? I suppose it's a matter of playing with the bit rate? and/or converting to some other format such as WMA?